From Fish Hooks Wiki

The Rat King is the king of the rats. Oscar and Milo were swimming in the Swirly Stadium when Oscar got flushed down the toilet and floated down the sewers below Bud's Pets, where he was captured by the Rat King. Angela also was flushed down into the sewers and got captured, but the Rat King let both of them go because they were boyfriend and girlfriend. He also invited both of them to a game of Mazes & Magic to be held the next week.